http://www.etonline.com/tv/161648_pretty_little_liars_postmortem_marlene_king_talks_a_shocking_secret_and_spoiler_return/

"We had three or four ideas of who we thought would be a great candidate to steal the game as 'A' from Mona," says I. Marlene King, "and then vey early on in season three we decided who that character is."